1. Field of the Invention
The present invention generally relates to a signal processing system and method in a DVD (Digital Video Disk) player. In particular, the invention relates to a system and method for decoding the data read out and relieved of a copy-protect at the front end and then performing a high quality and high speed playback of a sub-picture at the backend, which is connected to a television set, of a DVD player.
2. Description of the Related Art
In a conventional TV or VCR, information regarding sub-pictures, which are for example subtitles of a movie or lyrics of a karaoke song, is not separated from that of main pictures. In the DVD Standard decided upon in September, 1996, however, main pictures are encoded into a hierarchy of Video_Stream of the MPEG (Moving Picture Experts Group) Standard whereas sub-pictures are separated from the main picture information and stored into Private_Stream of the MPEG Standard as an independent form of a sub-picture unit (SPU). By constructing the sub-picture unit as a unit independent of the main pictures, it is possible to display multi-language subtitles which can be selected by the user and overlapping interactive sub-pictures. A sub-picture is said to be interactive when the sub-title can be controlled with respect to its color, contrast and display location in response to a selection made by a user.
Since a main picture and a sub-picture are independently stored in different streams, it is necessary to perform playback synchronously both between the main and sub pictures and between the video and audio data. In particular, when the DVD sub-picture is played back, the synchronization technique for special playbacks such as rapid and slow playbacks characteristics of the storage medium is critical.
In the DVD, the SPU's consist of a series of units. Each sub-picture unit comprises a PTS (Presentation Time Stamp) which specifies the time to start the decoding, an SPUH (Sub-Picture Unit Header) which shows the configuration of the unit, an SP_DCSQT (Sub-Picture Display Control Sequence Table) which is a control code consisting of a plurality of instruction sets each of which has its own execution time, and a PXD (Pixel Data) which is a bit map data to be displayed in response to an instruction from the instruction set from the SP_DCSQT. A PTS is a time stamp which provides an information control data The instruction sets that make up an SP_DCSQT are referred to as SP_DCSQT1, SP_DCSQT2, . . . , SP_DCSQTN, SP_DCSQTN+1, . . . , respectively.
The display of a sub-picture unit in ordinary playback as distinct from rapid or slow playbacks is processed as follows. The STC (System Time Clock) supplies the synchronous signal which is used by the sub-picture unit decoder as the reference. When the value of the STC reaches that of the PTS, then the SPU unit decoder clears the currently displayed SPU video data and executes the first instruction set SP_DCSQ0 of the SP_DCSQT. The nth instruction set is, as indicated above, represented as SP_DCSQN. The execution time SP_DCSQ_STM_N of this nth instruction set SP_DCSQN is individually defined and when this execution time is reached, the instruction is executed. Since the execution time SP_DCSQ_STM.sub.-- 0 of the first SP_DCSQ0 must be the same as that of PTS, however, the instruction must be immediately executed once the PTS time is reached.
Each instruction set contains the indication of the storage location, display location, display color, contrast, display beginning, and display end of the PXD. The DVD Standard requires that SP_DCSQ0 contain at least the minimum instructions necessary for the SPU data to be displayed. This is because it is assumed in the DVD Standard that the data only contained in this SP_DCSQ0 is sequentially decoded.
In the SPDCSQ, after certain options (color, contrast, display location, etc. ) are set, sub-picture data will be successively read out from the specified storage location of the PXD and displayed on the display monitor. When the decoder executes the instruction of SP_DCSQN, the decoder repeats to execute the above instruction until the STC reaches the execution time SP_DCSQ_STM_N+1 of the next SP_DCSQN+1. The DVD Standard requires that this instruction be executed synchronously with the vertical synchronous signal VSYN.
Since the DVD Standard further requires that the time interval of SP_DCSQ_STM be more than one video frame, one set of the PXD is necessarily executed when one sub-picture unit display beginning instruction is executed. However, in the case of the rapid playback where the STC itself is sped up by the rapid increment operation, more than one SP_DCSQT_STM exists during one video frame time. Since the SP_DCSQ is defined, in the DVD Standard, as an update from the result of the SP_DCSQ at the last execution time, it is not allowed to skip the SP_DCSQ. Accordingly, even if the STC is far ahead, commands have to be executed one by one. This is another reason why rapid execution is difficult to accomplish.
In order to overcome this difficulty, the DVD Standard assumes that only the first instruction of each sub-picture unit is executed. Thus, as already mentioned above, all the minimum instructions (color, contrast, display location, etc.) that are needed to display the SPU data must be included in SP_DCSQ0. However, in such a conventional rapid playback method that the DVD Standard originally assumes, the upgrade interval of the display depends upon the stream which is the time interval of each unit. It is difficult for the sub-picture to be smoothly played back and the display tends to be irregular because such a time interval is not constant and the length of the stream is variable. Further, since the update depends upon the stream, it is impossible for the update and display to be precisely synchronized, thereby making the display timing between the main and sub pictures imprecise. Additionally, since only the first instruction SP_DCSQ0 of the SPU is executed, all the interim commands existing up to the beginning of the next unit are ignored and abandoned, so that the video data displayed on the screen will appear to be a series of discontinuous frames, thereby resulting in unsmooth playback.